Catalytic in situ combustion



Patented July 2, 1963 3,095,925 CATALYTIC IN SITU COMBUSTION John W. Marx, Bartlesville, kla., assignor to Phillips Petroleum Company, a corporation of Delaware No Drawing. Filed July 20, 1959, Ser. No. 828,029 6 Claims. (Cl. 166-11) This invention relates to a process for the catalytic in situ combustion of carbonaceous strata. A specific aspect of the invention pertains to a process for depositing an ironcontaining catalyst in carbonaceous strata.

In situ combustion in the recovery of hydrocarbons from underground strata containing carbonaceous material is becoming more prevalent in the petroleum industry. In this technique of production, combustion is initiated in the carbonaceous stratum and the resulting combustion zone is caused to move thru the stratum by either inverse or direct air drive whereby the heat of combustion of a substantial proportion of the hydrocarbon in the stratum drives out and usually upgrades a substantial proportion of the unburned hydrocarbon material.

The ignition of carbonaceous material in a stratum around a borehole therein followed by injection of air thru the ignition borehole and recovery of product hydrocarbons and combustion gas thru another borehole in the stratum is a direct air drive process for effecting in situ combustion and recovery of hydrocarbons from the stratum. In this type of operation the stratum usually plugs in front of the combustion zone because a heavy viscous liquid bank of hydrocarbon collects in the stratum in advance of the combustion zone which prevents movement of air to the combustion process. To overcome this difficulty and to permit the continued progress of the combustion zone thru the stratum, inverse air injection has been resorted to. By this technique, a combustion zone is established around an ignition borehole by any suitable means and air is fed thru the stratum to the combustion zone from one or more surrounding boreholes.

In situ combustion techniques are being applied to tar sands, shale, Athabasca sand and other strata in virgin state, to coal veins by fracturing, and to strata partially depleted by primary and even secondary and tertiary recovery methods.

Iron is a well known combustion catalyst but it is difiicult to introduce iron salts in aqueous solution into subterranean strata because of the precipitation of iron compounds as the solution is being injected thru the pores of a stratum thereby plugging the stratum and interfering with the in situ combustion process to be performed, later. Ordinarily, iron salts in aqueous solution react with the carbonate material present in most reservoirs to produce formation-plugging precipitates around the injection well. This invention provides a method of introducing iron catalyst into a stratum without plugging same.

Accordingly, it is an object of the invention to provide a process for depositing an iron-containing catalyst in a carbonaceous stratum for use in effecting in situ combustion of the carbonaceous material. Another object is to provide a process for depositing such a catalyst in a carbonaceous stratum around an injection well therein without plugging the stratum. A further object is to provide an improved in situ combustion process in which the temperature is lower and the combustion rate is more rapid than is possible without the invention. It is also an object of the invention to provide an in situ combustion process for producing a carbonaceous stratum which produces more hydrocarbons in less time than is effected without the invention. Other objects will become apparent upon consideration of the accompanying disclosure.

A broad aspect of the invention comprises preparing an aqueous milky suspension of iron carbonate, thereafter introducing CO into the suspension so as to form a clear yellow solution, maintaining this solution under CO pressure so as to prevent precipitation of iron carbonate, injecting this clear yellow solution under said pressure into a substantial section of carbonaceous stratum to be produced, thereafter releasing the pressure on the solution so as to permit escape of C0 and precipitation of iron carbonate, and thereafter driving remaining aqueous liquid from the section of stratum to dry same and leave a deposition of iron carbonate therein. The concentration of iron to be used in the solution is in the range of 0.1 to 5 percent by weight (calculated as re,o, Concentrations as low as 0.1 weight percent are effective in reducing the temperature of the combustion in the stratum and corn centrations above about 5 percent are difficult to produce and do not appear to have any more catalytic effect than considerably lower concentrations.

To prepare the solution for injection into a stratum, iron carbonate is stirred into water to form a white, cloudy, or milky suspension of the carbonate, and CO is dissolved in the aqueous suspension in any suitable manner with CO vapor pressure applied to the liquid. As the CO is added, the milky suspension becomes a clear yellow solution which quickly reverts to a milky suspension of iron carbonate precipitate in Water upon release of the pressure and escape of CO from the liquid. Only sufficient CO pressure is required to prevent escape of CO from the liquid and precipitation of the carbonate. The prepared solution is injected into the selected stratum to be produced thru an injection well. The solution may be pumped down a well tubing while maintaining adequate CO pressure in the annulus to prevent precipitation during the injection phase of the process. If the section of stratum to be impregnated with a catalyst contains formation water the injected solution displaces the formation water radially outwardly from the injection borehole and the injection is continued until the desired section of stratum is filled with the solution; thereafter, the CO pressure is released so that CO escapes from the injected solution and allows precipitation of iron carbonate which deposits on the walls of the pores in the stratum. The remaining liquid is then forced out of the stratum by injecting a suitable gas, such as air, thereby drying out the stratum preparatory to the ignition step.

The section of stratum to be produced may comprise the stratum between a central ignition well and a ring of surrounding wells, in which case the injection of solution is preferably effected thru the central ignition well with water and air being produced thru the wells in the ring. The ignition is initiated around the central well by any suitable means, such as by burning a fuel pack in the ignition well adjacent the stratum and driving a combustion zone into the stratum by direct or inverse drive in conventional manner. The combustion front may then be driven thru the stratum by injecting combustion supporting gas thru the ignition borehole (direct drive) or by injecing the gas thru the Wells in the ring (inverse drive). In the first case, the produced hydrocarbons are recovered thru the wells in the ring, while in the second case, produced hydrocarbons are recovered thru the central ignition well.

The section of stratum to be ignited and produced by in situ combustion may comprise the intervening stratum between a line of ignition wells and two parallel lines of wells, one on each side of the line of ignition wells. The solution may be injected thru the line of ignition wells, which is preferred, or thru the lines of offset wells until solution appears in the line(s) of wells opposite the wells being used as injection wells. After release of CO pressure and deposition of iron carbonate in the intervening stratum, the remaining aqueous liquid is blown out of the section of stratum in any suitable manner by air drive. Combustion is then initiated along the line of ignition a,oo5,925

wells, preferably by igniting alternate Wells in the line and injecting air thru the remaining wells so as to move the combustion front across the intervening strip of stratum. When the combustion zone is established completely along the line of wells, combustion fronts may then be driven by direct or inverse drive across the intervening strips to the lines of offset wells.

Tests have shown that iron carbonate thus deposited in a carbonaceous stratum reduces the ignition and combustion temperature and also accelerates the fire propagation rate. By reducing the combustion temperature, substantially more oil or other hydrocarbon material may be produced from a given reservoir because the hundreds of thousands or millions of tons of rock in the reservoir are heated to a lower temperature, thereby requiring less fuel consumption than with a higher temperature. Since the temperature of combustion is reduced as much as about 70 to 100 F. by the use of the iron catalyst, the saving in fuel and increase in production of hydrocarbons from a reservoir is a substantial economic advantage. The process of the invention not only produces more oil but does so in a shorter period of time than without the use of the iron catalyst.

To illustrate the effects of the invention, tar sand from the Sulfur, Oklahoma, area was produced with and without the added iron catalyst of the invention. With deposi tion of iron carbonate in the sand prior to ignition, the average flame velocity was 0.532 ft./hr. while without deposition of a catalyst in the sand the average flame velocity was 0.47 ft./hr. In both instances, the air space velocity was 300 ft./hr. per square foot of cross section of sand. With catalyst present the average combustion temperature was 903 F., while without the catalyst in the sand the combustion temperature ranged from 970 to 1000 F.

In reservoirs containing an appreciable amount of connate or formation water, injection of a massive slug of iron-bearing CO solution under CO pressure into the pay zone prior to dry-out air injection, moves thru the porous sand or rock and is gradually diluted by formation water. This decreases the CO concentration with gradual deposition of iron carbonate on the iron surface, as the slug of solution is driven thru the stratum. The slug of solution may be followed by air injection which drives the remaining aqueous liquid out of the stratum to be produced and dries out the same for the in situ combustion step.

During the in situ combustion phase of the process the iron carbonate probably decomposes to iron oxide and there may even be some formation of elemental iron; however, the invention is not predicated upon any theory as to the form in which the iron exists at the time of its catalytic efiect upon the combustion process.

Three tests were made on three different reservoir rock or strata to determine the effect of the deposition of iron carbonate on the permeability thereof. In the first test the concentration of the iron compound in the solution was 1 /2 weight percent and the deposition thereof reduced the permeability from 154 md. (millidarcies) to 141 md. In the second and third tests the concentration of the iron carbonate in the solution was 1 percent by weight. In the second sample the permeability was reduced from 212 to 194 md. and in the third test applied to a less permeable stratum, the permeability was reduced from 85 to 63 md. These tests clearly indicate deposition of iron carbonate in the rock or sand without plugging the same or reducing the permeability sufficiently to prevent production of the strata by in situ combustion.

Certain modifications of the invention will become apparent to those skilled in the art and the illustrative details disclosed are not to be construed as imposing unnecessary limitations on the invention.

I claim:

1. A process for producing by in situ combustion a permeable carbonaceous stratum penetrated by at least first and second wells, comprising injecting into a section of said stratum thru one of said wells an aqueous solution of iron carbonate containing CO under CO pressure; reducing the pressure of said solution in said one of said wells after injection so as to allow CO; to escape from said section and said well, thereby effecting deposition of iron carbonate in said section; thereafter passing drying gas thru said section from well to well so as to drive out remaining aqueous liquid and dry said section; thereafter igniting said section adjacent one of said wells and feeding combustion-supporting gas to the ignited section so as to move the resulting combustion from therethru at a lower temperature and at a faster rate than would occur in the absence of said iron carbonate; and recovering the hydrocarbons produced by the combustion from one of said wells.

2. The process of claim 1 wherein said aqueous solution is injected thru an ignition well so as to drive water out of a section of stratum surrounding said well; and said combustion front is advanced from said ignition well to a ring of surrounding wells.

3. The process of claim 1 wherein said aqueous solution is injected thru a line of ignition wells to a parallel line of wells on each side of said line of ignition wells to drive water from the intervening stratum; and combustion fronts are driven from said line of ignition wells to each parallel line of wells.

4. A process for depositing an iron-containing catalyst in a permeable carbonaceous stratum prior to producing same by in situ combustion, which comprises preparing an aqueous milky suspension of iron carbonate containing iron (calculated as Fe O in the range of 0.1 to 5% by weight; introducing CO into said suspension so as to form a clear yellow solution; maintaining said solution under CO pressure so as to prevent precipitation of iron carbonate; injecting said clear yellow solution thru an injection well in said stratum under pressure into a substantial section of said stratum around said well; thereafter releasing pressure on said solution in said well so as to permit escape of CO and precipitation of iron carbonate in said section; and thereafter driving remaining aqueous liquid from said section by displacing said liquid with drying gas injected thru a well in said stratum to dry and impregnate said section with iron carbonate.

5. The process of claim 4 including the steps of igniting said section of stratum after drying same; moving a combustion front therethru by feeding combustion-supporting gas thereto thru a Well therein; and recovering the hydrocarbons produced by the combustion step from a well therein.

6. A process for producing hydrocarbons from a permeable carbonaceous stratum by in situ combustion com prising injecting into a section of said stratum thru a first bore hole therein an aqueous Solution of iron carbonate containing CO and under CO pressure; reducing the pressure on said solution after injection so as to allow escape of CO therefrom and effect deposition of iron carbonate in said section; thereafter driving out remaining aqueous liquid to dry said section by injecting air into said section thru a borehole in said stratum adjacent said section; thereafter igniting said section adjacent said first borehole and moving the resulting combustion front thru said section toward at least one offset borehole therein by passing combustion-supporting gas thereto thru a borehole therein, whereby the iron carbonate in said section increases the rate of combustion; and recovering hydrocarbons produccd by the combustion thru one of said boreholes.

References Cited in the file of this patent UNITED STATES PATENTS 

4. A PROCESS FOR DEPOSITING AN IRON-CONTAINING CATALYST IN A PERMEABLE CARBONACEOUS STRATUM PRIOR TO PRODUCING SAME BY IN SITU COMBUSTION, WHICH COMPRISES PREPARING AN AQUEOUS MILKY SUSPENSION OF IRON CARBONATE CONTAINING IRON (CALCULATED AS FE2O3) IN THE RANGE OF 0.1 TO 5% BY WEIGHT; INTRODUCING CO2 INTO SAID SUSPENSION SO AS TO FORM A CLEAR YELLOW SOLUTION; MAINTAINING SAID SOLUTION UNDER CO2 PRESSURE SO AS TO PREVENT PRECIPITATION OF IRON CARBONATE; INJECTING SAID CLEAR YELLOW SOLUTION THRU AN INJECTION WELL IN SAID STRATUM UNDER PRESSURE INTO A SUBSTANTIAL SECTION OF SAID STRATUM AROUND SAID WELL; THEREAFTER RELEASING PRESSURE ON SAID SOLUTION IN SAID WELL SO AS TO PERMIT ESCAPE OF CO2 AND PRECIPITATION OF IRON CARBONATE IN SAID SECTION; AND THEREAFTER DRIVING REMAINING AQUEOUS LIQUID FROM SAID SECTION BY DISPLACING SAID LIQUID WITH DRYING GAS INJECTED THRU A WELL IN SAID STRATUM TO DRY AND IMPREGNATE SAID SECTION WITH IRON CARBONATE. 